This invention relates to microelectronic device structures, and, more particularly, to increasing the thermal conductivity of the package flange that supports the microelectronic device.
A microelectronic device typically has a large number of active and passive circuit elements formed in a semiconductor material, the unitary structure being termed a die or chip. The die is small in size and relatively fragile. It is therefore affixed to and supported on a support structure which includes a package flange and, optionally, a cover. The support structure physically supports and protects the die, and provides the points of electrical interconnection to the circuit elements on the die.
The removal of heat from microelectronic devices, particularly from those that process high-power signals, is often the limiting factor in their utilization. If a sufficient quantity of heat is not removed, the temperature of the microelectronic device rises and eventually exceeds its operating limits. In the usual case, the support structure provides a conductive heat flow path for removal of heat from the die. A number of different support materials and support structure designs have been developed to maximize the removal of heat from the microelectronic device and through the support structure.
Chemical vapor deposited (CVD) diamond has been identified as a particularly useful material of construction for the support structure, because of its high thermal conductivity. However, the CVD diamond is expensive, and usually constitutes only a portion of the support structure directly under the die of the microelectronic device. That is, the CVD diamond portion of the support structure is bonded to the remainder of the support structure, and the microelectronic device is mounted to the CVD diamond portion.
While this approach is operable, it has drawbacks. Differential thermal strains and stresses between the remainder of the support structure, the diamond portion of the support structure, and the microelectronic device die can adversely affect the operation of the microelectronic device and/or lead to debonding. The interface between the diamond and the remainder of the support structure constitutes a thermal impedance in the heat flow away from the microelectronic device, and a high-conductivity interface may be difficult to achieve in the case of diamond.
There is a need for an improved approach to heat removal from microelectronic device dies in package structures. The present invention fulfills this need, and further provides related advantages.
The present invention provides a microelectronic device structure having a package flange with improved thermal properties to remove heat from a supported electronic device. A composite diamond structure is used, but differential thermal strains and stresses are reduced as compared with prior approaches. Fabrication of the package flange with precision alignment is readily achieved.
In accordance with the invention, a microelectronic device structure comprises a package flange having a body with a body upper surface, a substantially circular body interior sidewall defining an opening in the body upper surface, and a substantially circular inlay comprising diamond, preferably chemical vapor deposited (CVD) diamond. The inlay is received into the substantially circular opening of the body upper surface, and the inlay has an inlay exterior sidewall that is adjacent to, and preferably contacting in places, the body interior sidewall. Preferably, the inlay has an inlay upper surface which is substantially coplanar with the body upper surface. In service, a microelectronic device is typically affixed to the inlay upper surface.
In one embodiment, the body comprises a planar shim and a planar base contacting and affixed to the shim. The body interior sidewall extends through the base but not through the shim. Preferably, the shim is copper and the base is copper-tungsten or Silvar(trademark) material.
To facilitate fabrication and minimize differential thermal strains and stresses, the inlay exterior sidewall is preferably tapered inwardly with increasing distance from an inlay upper surface. The inlay is press fit into the opening, and then the inlay exterior sidewall is affixed to the body interior sidewall with a joining material.
The present approach uses a circular diamond inlay, rather than a diamond inlay of another shape. The use of the circular inlay maximizes the sidewall surface area through which heat is conducted for any selected upper surface area. The circular shape also minimizes the thermal stresses in the relatively brittle inlay, due to the short moment arm to the inlay exterior sidewall and the relatively large shear volume in the inlay.
Because circular inner and outer sidewalls may usually be cut more accurately to precision tolerances than other shapes, and the inner sidewall may be reamed to a precise dimension, the present approach permits the fabrication of the body and the inlay more readily than if another shape were used. The result is more accurate alignment of the body and the inlay, and a closer planarity of the upper surfaces of the body and the inlay.
During assembly, the inlay is press fit into the opening of the body, producing radially inwardly directed compressive stresses in the body that hold the inlay in place during subsequent brazing. The inwardly directed compressive stresses in the shim aid in maintaining a good contact at the circular interface between the inlay and the shim.
In the preferred embodiment wherein the body has two layers, the shim of which is soft copper, the copper may be easily planarized after the press fitting and brazing. The copper shim is easily made planar by sanding or other comparable operation.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.